Go-garbage-collection

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func main() {
	userInfo := elvis.userINfo    // ===>读屏障
	jj.Info = getUserInfo.       //  ===>写屏障
}

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func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
    ...
    shouldhelpgc := false
    // 分配的对象小于 32K byte
    if size <= maxSmallSize {
        ...
    } else {
        shouldhelpgc = true
        ...
    }
    ...
    // gcShouldStart() 函数进行触发条件检测
    if shouldhelpgc && gcShouldStart(false) {
        // gcStart() 函数进行垃圾回收
        gcStart(gcBackgroundMode, false)
    }
}

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// GC runs a garbage collection and blocks the caller until the
// garbage collection is complete. It may also block the entire
// program.
func GC() {
    gcStart(gcForceBlockMode, false)
}

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// gcShouldStart returns true if the exit condition for the _GCoff
// phase has been met. The exit condition should be tested when
// allocating.
//
// If forceTrigger is true, it ignores the current heap size, but
// checks all other conditions. In general this should be false.
func gcShouldStart(forceTrigger bool) bool {
    return gcphase == _GCoff && (forceTrigger || memstats.heap_live >= memstats.gc_trigger) && memstats.enablegc && panicking == 0 && gcpercent >= 0
}

//初始化的时候设置 GC 的触发阈值
func gcinit() {
    _ = setGCPercent(readgogc())
    memstats.gc_trigger = heapminimum
    ...
}
// 启动的时候通过 GOGC 传递百分比 x
// 触发阈值等于 x * defaultHeapMinimum (defaultHeapMinimum 默认是 4M)
func readgogc() int32 {
    p := gogetenv("GOGC")
    if p == "off" {
        return -1
    }
    if n, ok := atoi32(p); ok {
        return n
    }
    return 100
}

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// gcStart starts the GC. It transitions from _GCoff to _GCmark (if
// debug.gcstoptheworld == 0) or performs all of GC (if
// debug.gcstoptheworld != 0).
//
// This may return without performing this transition in some cases,
// such as when called on a system stack or with locks held.
func gcStart(trigger gcTrigger) {
	...
	mode := gcBackgroundMode
  //如果forceMode则直接跳转到_GCmarktermination
	if debug.gcstoptheworld == 1 {
		mode = gcForceMode
	} else if debug.gcstoptheworld == 2 {
		mode = gcForceBlockMode
	}
	...
  //启动MarkStartWorkers的goroutine
	gcBgMarkStartWorkers()
  // Stop The World
	systemstack(gcResetMarkState)
   ...
	//处理设置 GCPhase，setGCPhase 还会 enable write barrier
	setGCPhase(_GCmark)

	gcBgMarkPrepare() // Must happen before assist enable.
	gcMarkRootPrepare()

	gcMarkTinyAllocs()
	...
}

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func gcBgMarkStartWorkers() {
	for gcBgMarkWorkerCount < gomaxprocs {
    // mark方法.  ==<看这里
		go gcBgMarkWorker()

		notetsleepg(&work.bgMarkReady, -1)
		noteclear(&work.bgMarkReady)
		gcBgMarkWorkerCount++
	}
}

// gcBgMarkWorker 是一直在后台运行的，大部分时候是休眠状态，通过 gcController 来调度
func gcBgMarkWorker() {
	...
	for {
		// Go to sleep until woken by
		// gcController.findRunnableGCWorker.
		gopark(func(g *g, nodep unsafe.Pointer) bool {
			...//一些系统协程调度优化

			// gcBgMarkWorkerPool 优化
			gcBgMarkWorkerPool.push(&node.node)
		 ...
		//// mark 过程
		systemstack(func() {
			// Mark our goroutine preemptible so its stack
			// can be scanned. This lets two mark workers
			// scan each other (otherwise, they would
			// deadlock). We must not modify anything on
			// the G stack. However, stack shrinking is
			// disabled for mark workers, so it is safe to
			// read from the G stack.
			casgstatus(gp, _Grunning, _Gwaiting)
			switch pp.gcMarkWorkerMode {
			default:
				throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
			case gcMarkWorkerDedicatedMode:
				gcDrain(&pp.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
				if gp.preempt {
					// We were preempted. This is
					// a useful signal to kick
					// everything out of the run
					// queue so it can run
					// somewhere else.
					lock(&sched.lock)
					for {
						gp, _ := runqget(pp)
						if gp == nil {
							break
						}
						globrunqput(gp)
					}
					unlock(&sched.lock)
				}
				// Go back to draining, this time
				// without preemption.
				gcDrain(&pp.gcw, gcDrainFlushBgCredit)
			case gcMarkWorkerFractionalMode:
				gcDrain(&pp.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
			case gcMarkWorkerIdleMode:
				gcDrain(&pp.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
			}
			casgstatus(gp, _Gwaiting, _Grunning)
		})

		// Account for time.
		...

		// Was this the last worker and did we run out
		// of work?
		...
			gcMarkDone()
		}
	}
}

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func gcDrain(gcw *gcWork, flags gcDrainFlags) {
	if !writeBarrier.needed {
		throw("gcDrain phase incorrect")
	}
  ...//一些markingWorker的提取与检查
	// Drain root marking jobs.
	if work.markrootNext < work.markrootJobs {
		// Stop if we're preemptible or if someone wants to STW.
		for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
			job := atomic.Xadd(&work.markrootNext, +1) - 1
			if job >= work.markrootJobs {
				break
			}
			markroot(gcw, job)
			if check != nil && check() {
				goto done
			}
		}
	}

	// Drain heap marking jobs.
	// Stop if we're preemptible or if someone wants to STW.
	for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
		if work.full == 0 {
			gcw.balance()
		}
		//获取灰色队列中的对象
		b := gcw.tryGetFast()
		if b == 0 {
			b = gcw.tryGet()
			if b == 0 {
				// Flush the write barrier
				// buffer; this may create
				// more work.
				wbBufFlush(nil, 0)
				b = gcw.tryGet()
			}
		}
		if b == 0 {
			// Unable to get work.
			break
		}
    //扫描灰色对象的引用对象，标记为灰色，入灰色队列
		scanobject(b, gcw)
	...

}

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func gcMarkTermination() {
    // World is stopped.
    // Run gc on the g0 stack. We do this so that the g stack
    // we're currently running on will no longer change. Cuts
    // the root set down a bit (g0 stacks are not scanned, and
    // we don't need to scan gc's internal state).  We also
    // need to switch to g0 so we can shrink the stack.
    systemstack(func() {
        gcMark(startTime)
        // Must return immediately.
        // The outer function's stack may have moved
        // during gcMark (it shrinks stacks, including the
        // outer function's stack), so we must not refer
        // to any of its variables. Return back to the
        // non-system stack to pick up the new addresses
        // before continuing.
    })
    ...
}

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func bgsweep(c chan int) {
    sweep.g = getg()

    lock(&sweep.lock)
    sweep.parked = true
    c <- 1
    goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)

    for {
        for gosweepone() != ^uintptr(0) {
            sweep.nbgsweep++
            Gosched()
        }
        lock(&sweep.lock)
        if !gosweepdone() {
            // This can happen if a GC runs between
            // gosweepone returning ^0 above
            // and the lock being acquired.
            unlock(&sweep.lock)
            continue
        }
        sweep.parked = true
        goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)
    }
}

func gosweepone() uintptr {
    var ret uintptr
    systemstack(func() {
        ret = sweepone()
    })
    return ret
}

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// sweeps one span
// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
func sweepone() uintptr {
    ...
    for {
        s := mheap_.sweepSpans[1-sg/2%2].pop()
        ...
        if !s.sweep(false) {
            // Span is still in-use, so this returned no
            // pages to the heap and the span needs to
            // move to the swept in-use list.
            npages = 0
        }
    }
}

// Sweep frees or collects finalizers for blocks not marked in the mark phase.
// It clears the mark bits in preparation for the next GC round.
// Returns true if the span was returned to heap.
// If preserve=true, don't return it to heap nor relink in MCentral lists;
// caller takes care of it.
func (s *mspan) sweep(preserve bool) bool {
    ...
}